Unique Features to Nematoda
This is a normal nematode. Image with permission of Dr. Tietjen at Bellarmine University from http://cas.bellarmine.edu/tietjen/images/phylum_nematoda.htm.
Nematodes are often confused with other closely related types of worms. These are often part of the phylum Platyhelminthes and are known as flatworms because they lack a body cavity. Similarly, Annelids can sometimes be confused with Nematodes but are distinguishable because they have a true coelom (Phil Myers, "Phylum Annelida" 2004).
Roundworms have a thick cuticle that covers their bodies and is shed in order to allow for growth. Located between the cuticle and the pseudocoel are muscles that run the length of the nematode. These muscles push on both the cuticle and the pseudocoel and create a kind of hydrostatic skeleton (Waggoner 2004). In contrast with most animals whose nerve cells branch out to each individual muscle cell, nematode’s muscle cells branch toward the nerve cells (Waggoner 2004).
Nematode cross section in the gut region. Image obtained courtesy of BIODIDAC: http://www.ucmp.berkeley.edu/phyla/ecdysozoa/nematoda.html
All nematodes do have a simple but defined digestive tract.
A roundworm mouth usually has 16 protruding sensory organs and sometimes
displays piercing structures called “stylets.”
Food goes straight through the conveyor belt-like tract and is broken
down, diluted with water, absorbed, and excreted.
Unlike most animals, nematodes do not depend on cilia or flagella for
excretion. Rather they utilize cells
that work as glands or systems of canals in order to get rid of waste (Raven et
al. 745).
Nematodes are sexual animals, and the male is generally slightly smaller than the female, which usually displays a bent tail. Nematode reproduction in free-living specimensis a very interesting process involving six stages including an egg stage, four larval stages (L1, L2, L3, L4), and an adult stage. Males are diecious in that they can have one or two testes and can have a variety of accessory sex organs depending on the species. Females give rise to eggs that are then fertilized and laid. Once the embryos in these eggs are mature they will hatch into the L1 larval stage. The juvenile nematode will then undergo four molts before it becomes an adult and is capable of reproduction. During molting, a nematode will shed its skin in order to facilitate growth. The third L3 larval stage is normally the infectious stage for parasitic nematodes ("Nematoda" 2004). The diagram below illustrates the nematode life cycle.
Diagram obtained from http://martin.parasitology.mcgill.ca/jimspage/biol/nema.htm
Parasitic nematode life cycles vary more than those of free-living specimens. Often parasitic roundworms will have multiple stages and alternate between hosts and regions in their hosts' bodies.
Evolution & Phylogenetic Relationships
Features Shared by Nematodes with Related Groups